Field of the Invention
Embodiments of the present invention relate to an apparatus and a method for measurement of properties of a ferromagnetic material, for example for measuring stress.
Description of the Related Art
The stresses in structures such as rails, bridges and pipelines, complex mechanisms such as vehicles and machinery, or simple devices such as struts, cables or bearings arise from various causes including changes of temperature, and the loads and pressures due to use. There may also be residual stresses arising from the fabrication of the structure or device. In some situations (such as pipelines) the principal stress directions can be expected to be in particular directions (circumferential and longitudinal), whereas in other situations the principal stress directions are also unknown. A variety of magnetic techniques are known to have some sensitivity to stress, although magnetic measurements are usually also affected by other material properties such as microstructure. A way of measuring stress in a steel plate is described in GB 2 278 450, this method using a probe containing a U-shaped electromagnetic core with two spaced-apart poles and with a drive coil wound around the middle of the core to generate an alternating magnetic field in the plate, and then combining measurements from two sensors, one being a measure of stress-induced magnetic anisotropy, and the other being a measure of directional effective permeability (DEP). The latter is sensed by a sensing coil that is also wound around the middle of the core. Such electromagnetic measurements are affected not only by material properties, but also by geometrical factors, in particular the lift off from the surface (i.e. the gap between the probe and the surface). Ways of eliminating the effect of lift off are described in EP 1 436 604=WO 03/034054, describing both a graphical and an algebraic procedure.
A more sensitive way of sensing material properties, such as stress, would be desirable.